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Manure from a high-density cattle farm that holds upward of 100,000 cows may have been the source of a deadly Escherichia coli strain that found its way onto romaine lettuce and caused a massive outbreak earlier this year. That’s according to a new hypothesis announced this week by the Food and Drug Administration.

The bacterium behind the outbreak was a particularly nasty strain of Shiga toxin-producing Escherichia coli O157:H7 that produces only Shiga toxin type 2 (Stx2), the more toxic of two types of toxins E. coli tends to carry. Stx2 causes cell death, triggers immune responses, and leads to the destruction of red blood cells, which can damage the kidneys.

Such Shiga-toxin producing E. coli are shed from the guts of animals (particularly cattle) and are spread by feces.

Traceback investigations by federal authorities linked the illnesses to romaine lettuce grown in the Yuma region of Arizona. Further work found that the outbreak stain was present in canal water running along farms. That pointed experts to the idea that tainted canal water was used for irrigation, literally showering crops with deadly germs.

In the new update, the FDA notes that a Concentrated Animal Feeding Operation (CAFO) is located nearby to a cluster of romaine lettuce farms. Such high-density farms are notorious for causing water quality issues. Thus, poopy runoff from the CAFO may have contaminated the canal water, which then made its way onto vegetables directly through irrigation or some other indirect route. The FDA noted that it has been pondering other hypotheses, but it didn’t outline what those were.

“Our experts continue to work on examining potential links between the CAFO, adjacent water, and geologic and other factors that may explain the contamination and its relationship to the outbreak,” the agency said in its update. “Additional sampling activities will be conducted to further explore and narrow down hypotheses in the near future.”

The FDA plans on releasing a detailed environmental assessment report when it’s done with the investigation.